1. Field of the Invention
The present invention relates to a system for controlling the temperature distribution of a charged chip mass in a digester, such as a continuous digester for continuously making pulp.
2. Description of the Related Art
In a paper manufacturing plant, a continuous digester has conventionally been used as a digester. In the continuous digester, wood is broken or cut to small chips and pulp, a raw material for paper, is continuously manufactured from the small chips. In the continuous digester, the wood chips are heated or cooked with the use of an alkali solution and broken down into lignin and cellulose portions. By washing the cellulose portions, it is possible to obtain pulp, that is, the raw material for paper. The chips are supplied into the continuous digester from above with the chips closely packed in the digester, a resultant chip mass descends toward a lower zone, while being reacted with the alkali solution. The cellulose portion (pulp) is washed with a washing liquid fed from the bottom of the digester and is drawn from the digester bottom.
In this case, it is necessary that the pulp be comprised of a pure cellulose portion alone free from any lignin contents. Further, the longer the charged chip mass is heated at high temperature, the more the lignin is degraded. However, there occurs a loss in the cellulose portion, that is, a loss in the raw material either. It is, therefore, necessary to heat the chip mass at a suitable temperature for a proper time period only. In this case, it is to be noted that the extent of heating above is called a "heated" state, represented by an H factor.
It is, therefore, very important to measure the temperature distribution of the chip mass in the digester, that is, in such a continuous digester, and to control the temperature so that the charged chip mass in the digester may take on an appropriate temperature distribution. This is very important to achieve high quality, high efficiency and high resource savings in the making of paper.